High-Pressure Tungsten Bronzes, RExWO3 with RE=La and Nd, Studied by X-Ray Diffraction and Electron Microscopy

Rare earth tungsten bronzes, La0.1WO3 and NdxWO3 with x=0.1 and 0.2, have been prepared by solid state reaction at P=30 kbar and T=1320–1620 K and structurally characterized by X-ray diffraction and electron microscopy. The hexagonal tungsten bronze (HTB) structure was found in all samples, whereas intergrowth tungsten bronzes of (n)-ITB and (1, n)-ITB structure types have only been observed in samples prepared at T>1500 K. The ITB structures can be considered as an intergrowth of slabs of WO3-type, n octahedra wide, and slices of HTB-type. In the (n)-ITB family the slices of HTB-type are one hexagonal tunnel row wide, whereas they are two hexagonal tunnel rows wide in the (1, n)-ITB phases. Members of the (n)-ITB family were frequently observed in the high-temperature samples. The LaxWO3 bronzes, (2)-ITB, (3)-ITB, (1, 2)-ITB, and HTB, are new. In the NdxWO3 system a perovskite tungsten bronze (PTB) of composition Nd≈0.1WO3 and a (3)-ITB bronze were found in addition to the previously reported high-pressure phases (2)-ITB and HTB. The bronze structures were well-ordered, but a few exceptions were observed. A couple of defects are described along with proposed defect models. Microanalyses show that the hexagonal tunnel sites in the ITB and HTB structures are less than half-filled with RE atoms. The relationships between the RExWO3 bronzes and the alkali tungsten bronzes, AxWO3, are discussed.